1. Field of the Invention
The present invention relates to an improved process for producing an aqueous solution of calcium nitrite by contacting a nitrogen oxides containing gas which contains NO and NO.sub.2 with an aqueous slurry of calcium hydroxide to absorb the gas.
2. Description of the Prior Art
Heretofore, calcium nitrite has been used as an anti-corrosive agent and an additive for a cement etc. In industrial usages, calcium nitrite is preferably used in a form of an aqueous solution rather than a solid form, and especially in a form of an aqueous solution having about 30 to 40 wt. % of calcium nitrite.
The aqueous solution of calcium nitrite can be easily obtained by dissolving a solid calcium nitrite into water. The commercially available solid calcium nitrite is obtained by concentrating and drying an aqueous solution of calcium nitrite and accordingly, the method of dissolving the solid calcium nitrite is in remarkably low efficiency.
It has been proposed to produce an aqueous solution of calcium nitrite in Japanese Patent Publication No. 35596/1976. However, the known process requires many complicated steps such as filtration, an aging, a concentration, a second filtration, and a second concentration, etc. to be low efficiency and to give large loss of the starting materials of a nitrogen oxides gas and an aqueous slurry of calcium hydroxide, disadvantageously.
The inventors have studied to attain a process for producing an aqueous solution of calcium nitrite having high purity and high concentration from a nitrogen oxides containing gas and an aqueous slurry of calcium hydroxide, and have found the fact that a formation of a by-product of calcium nitrate can be prevented by contacting a gas having less than about 3 vol. % of a nitrogen oxides concentration with an aqueous slurry having 3 to 10 wt. % of a calcium hydroxide content and containing calcium nitrite to absorb it whereby calcium nitrite is produced at high efficiency.
The inventors have studied and found a process for producing an aqueous solution of calcium nitrite by (1') a step of contacting a nitrogen oxides containing gas having 5 to 10 vol. % of a nitrogen oxides concentration and 1.2 to 1.5 of a molar ratio of NO/NO.sub.2 with an aqueous slurry having 20 to 40 wt. % of a calcium hydroxide content at 40.degree. to 70.degree. C. until reducing the calcium hydroxide content in a range of 3 to 10 wt. % as a first stage and (2') a step of separating the unabsorbed and unreacted gas and oxidizing the separated gas to form a gas having 1 to 3 vol. % of a nitrogen oxides concentration and 1.2 to 1.5 of a molar ratio of NO/NO.sub.2 and (3') a step of contacting the resulting gas having low nitrogen oxides concentration with the separated aqueous slurry having 3 to 10 wt. % of a calcium hydroxide content and containing calcium nitrite at 40.degree. to 70.degree. C. to reduce the calcium hydroxide content to be less than 3 wt. % as a second stage and (4') a step of filtering the resulting solution having high calcium nitrite concentration.
In order to produce a large amount of an aqueous slurry of Ca(NO.sub.2).sub.2 having a high purity of higher than 95% at a high concentration of higher than 30% for example, a rate of several thousands tons per year, a large apparatus is required. In order to maintain the temperature of the reaction slurry at 40.degree. to 70.degree. C. in the absorption of a gas containing nitrogen oxides (a molar ratio of NO/NO.sub.2 of 1.2 to 1.5) at higher than 150.degree. C. obtained by the oxidation of ammonia into an aqueous slurry of slaked lime having a concentration of 20 to 40%, it is necessary to use a large cooler for cooling and a large reactor for oxidation of the discharged gas from the first reactor.
It is important to overcome these disadvantages.
The efficiency for cooling can be improved by raising the reaction temperature, however, when the reaction temperature is higher than 70.degree. C. under atmospheric pressure, a partial pressure of steam of the slurry is higher than the partial pressure of steam in the gas containing nitrogen oxides whereby a concentration of the slurry caused by evaporation of steam is resulted and the complex Ca(NO.sub.2).sub.2.Ca(OH).sub.2.2H.sub.2 O is precipitated to increase the viscosity of the slurry. Therefore, the reaction is not smoothly performed.
The inventors have further studied to prevent a concentration of the slurry and have found that calcium nitrate as a by-product is increased in the reaction and accordingly, an aqueous solution of calcium nitrite having high purity (higher than 95%) could not be obtained. On the other hand, the inventors have found the fact that even though a solubility of slaked lime is decreased depending upon raising the temperature, the reaction velocity is substantially the same. Moreover, the inventors have found the fact that the increase of calcium nitrate as a by-product and the concentration of the slurry can be prevented by absorbing a gas containing nitrogen oxides (the molar ratio of NO/NO.sub.2 is higher) under a higher pressure at a temperature of the slurry of higher than 70.degree. C.